1. Field of the Invention
The present invention relates to an ultrasonic blood flow imaging apparatus and more particularly to the improvement of such an apparatus which is capable of two-dimensionally in color displaying the velocity distribution of a moving member such as blood flow or the property of tissues within a living organism.
2. Description of the Prior Art
There are well known ultrasonic imaging apparatuses wherein an ultrasonic wave beam is directed into a subject to be examined and an image is formed using the reflected echo signals produced as a result of differences in acoustic impedance within the subject under examination. These ultrasonic imaging apparatuses have been practically applied to ultrasonic diagnostic apparatuses, ultrasonic Doppler diagnostic apparatuses and the like. These apparatuses are advantageous in that they make it possible to observe the interior of the subject without adversely affecting its structural make-up and are therefore used, for example, in visual diagnostic examination of afflicted tissues and organs in the human.
In such ultrasonic Doppler diagnostic apparatuses, the Doppler effect that arises when an ultrasonic pulse beam strikes the moving member such as blood flow in a body to be examined is used to determine the velocity of movement of blood flow. In the ultrasonic Doppler diagnostic apparatus, the velocity of the moving member is displayed as colored images. For example, see our U.S. Pat. No. 4,573,477.
Such an apparatus provides different colors and their hues corresponding to the forward and rearward directions or velocities of blood flow, respectively. For example, the flow of blood approaching a probe of the apparatus is displayed in red color while the flow of blood moving away from the probe is displayed in blue color. In addition to the color display, the velocity of blood flow is indicated with changes of brightness. Thus, two-dimensional distribution of velocity can be easily realized to provide color display of the moving member in the living body.
However, in the prior art ultrasonic diagnostic apparatuses the moving reflective member can be displayed in color only in the limited directions, that is, in the first direction in which the moving reflective member approaches the detecting probe and in the second direction in which the moving reflective member moves away from the detecting probe. This results from the fact that the orientation of the moving reflective member cannot accurately and promptly be determined in all the directions.
In recent years, proposals have been made which calculate the vector velocities of the moving reflective member to determine the velocities thereof in all the directions. For example, there has been proposed a method of irradiating a body to be examined with ultrasonic beams from two different directions and then determining vector velocities from the velocity components in the directions of the beams. Furthermore, a method of measuring the vector velocities by use of a single ultrasonic beam has been filed by the inventors simultaneously.
It is thus desired to provide an apparatus which makes it possible to clearly display in color the state of a moving reflective member in all the directions.
The prior art apparatuses have a further problem in that they cannot accurately display the properties of tissue, for example, such as the hardness, density and so on, because the apparatuses display mainly the configuration of the tissue about its boundaries.